Abstract Studies in our laboratory demonstrate that vitamin D (1,25 dihydroxycholecalciferol or calcitriol) has significant antitumor activity in vitro and in vivo in a variety of murine, rat and human tumor model systems. Calcitriol induces G0/G1 arrest, modulates p27 and p21, induces cleavage of caspase 3, PARP and MEK and decreases P-Erk and P-Akt. Calcitriol significantly enhances the in vitro and in vivo antitumor efficacy of the platinum analogues, taxanes and nucleosides in a number of murine/human xenograft tumor models and in a trial in spontaneous dog tumors. Enhancement of drug-mediated apoptosis by calcitriol is associated with an increase in apoptosis markers. In addition, the expression of the p53 homolog, p73, is strongly induced by calcitriol and p73 sensitizes tumor cells to the cytotoxic effects of cisplatin suggesting a central role for p73 in calcitriol/ cisplatin-mediated apoptosis. In bladder tumor model systems, we demonstrated that calcitriol up-regulates p73 in bladder tumor cell lines and synergistically enhances the antitumor activity of cisplatin and /or gemcitabine in vitro and in vivo in bladder models. Also, when p73 is down-regulated with siRNA-p73, calcitriol-mediated enhancement is not observed. Clinical studies have determined the MTD of calcitriol alone and in combination with a number of cytotoxic agents and/or glucocorticoids, where there is synergy pre- clinically. To correlate response in the clinic with calcitriol-induced molecular changes has been difficult due to the lack of access to tumor tissue in patients for analysis. We propose here to determine the mechanisms involved in calcitriol enhanced cisplatin-mediated death cell and to extend these studies from an animal model to a phase I study to determine dose, safety and toxicity and then to the neoadjuvant phase II setting where bladder cancer patients will be treated prior to surgery and where tumor material can be examined for molecular modulation. Therefore, we propose to test the hypothesis that calcitriol sensitizes tumor cells to chemotherapy-mediated cell death through modulation of p73 both pre-clinically and clinically by addressing the following specific aims: 1) to determine the role of p73 in calcitriol enhancement of cisplatin- mediated antitumor activity; 2) to determine the relationship between modulation of p73 in vitro and antitumor response in vivo in animal tumor model systems; 3) to examine in a phase I trial, the combination of cisplatin (75mg/m2, D2 every 28days) and gemcitabine (800mg/m2, D2 weekly) with escalating doses of calcitriol (starting at 0.25g, D1,2 weekly); and 4) to evaluate through the conduct of a neoadjuvant phase II clinical trial, the effect of calcitriol (phase I dose, D1, 2), cisplatin (75mg/m2, D2 every 28days) and gemcitabine (800mg/m2, D2 weekly) for 3 cycles before cystectomy in muscle-invading transitional cell bladder cancer.